2 He hasn’t seen his family _____ three years ago.

scince

此句是现在完成时（have/has+动词过去分词），后面加时间的话用for或scince。区别在，for后面加一段时间，而scince后面加过去的时间。因为这里有这个ago表示过去的词，所以用scince。也可以改为：He hasn't seen his family for three years.意思都一样，他有三年没有见到他的家人了。

since

since
自从三年前开始，他一直没有见过他的家人。
(= He hasn't seen his family for three years.)
也许你对“since + 时间点”比较熟悉，如：
He hasn't seen his family since last year.
Nothing has happened since then.
此外，since后面也可以用 ... ago. 再如：
I have left the door open since ten minutes ago.